A retro-modulation device with modulation capability, as disclosed herein, includes a compact, monolithic optical MEMS structure, which uses mechanical motion, induced electrostatically or thermally, to displace various components of the structure for the modulation encoding. In general, retroreflectors have the property of taking an incident optical (or RF) beam received from a source and redirecting the beam back to the source via a sequence of reflections within the retro-device As such, retro-reflective devices, in essence, “track” the source location and thus provide auto-compensation for beam wander, relative platform motion and wobble. In addition, by encoding a modulation signal at (or within) the retro-reflective device, the return beam can be encoded with the information from the modulation signal. In this manner, information can be directed from a remote site back to the location of an interrogation or source beacon without the need for an optical (or RF) source or transmitter at the remote site. Retro-reflection and modulation devices have myriad applications to free-space links, optical interconnects, remote sensors, IFF/ID battlefield scenarios, guidance control for free-space or underwater platforms, etc.
The retro-reflecting structures can be in the form of a corner cube or cat's eye configuration as well as an array of such devices (the latter is referred to as a pseudo-conjugator). In either case, in accordance with the present invention, a modulation signal is used to displace the element(s) of the structure to, in essence, reversibly “defeat” the retro-directive capability of the device. The result is that the return beam returns back to the transceiver location as an amplitude-modulated beam or as an on-off digitally or binary modulated beam, bearing the desired encoded temporal information. Since the mechanical displacement is cyclical (billions of cycles have been demonstrated using optical MEMS devices in general), modulation information can be relayed using such a device with high reliability over a very long period of time. Modulation bandwidth in the range of kHz to many MHz are possible using this scheme.
Large arrays of retro-reflecting structures can be utilized as disclosed in U.S. provisional patent application Ser. No. 60/420,177 noted above.
Both types of retro-reflection devices (cat's eye and corner cube structures) are known, per se, in the prior art. Additionally, the four variations of optical switches subsequently discussed herein have also been discussed in the prior art. The present invention merges the two notions (retro-reflection devices with optical switching methods) to form a retro-modulator. This concept is not apparent to persons skilled in the art, since the primary applications of the four classes of switches have been presented by experts in the field in the context of high-definition display-mode devices (spatial light modulators, SLMs), high-density optical interconnection elements for telecommunications networks, wavelength divisional multiplexing (WDM) optical devices of high-density communication traffic networks and optical spectrometers. The notion of combining any of these switching approaches for use as modulated retro-devices is not known in the prior art. Therefore, the present invention is novel and, as discussed above and below, has myriad uses and applications in a variety of fields and scenarios. The disclosed retro-modulator should find itself very useful in many applications. For example, the disclosed retro-modulators can be used for combat ID/IFF, remote sensor optical links, interconnects, free-space links to communicate with missiles, high-speed smart projectiles, underwater devices, UAVs, and micro-UAVs, etc., and for communication links in general (telecommunication, wireless, interconnects, etc.). The disclosed retro-modulators can be used in connection with inter vehicle communication and accident avoidance tagging.
One object of this invention is to make it possible to realize a compact remote communication device, with modulation capability. The advantages of this invention include:                1. The disclosed retro-modulation devices are preferably passive; therefore the optical interrogation source needs only to be located at the interrogation/beacon site, and not necessarily at the retro-device location.        2. The disclosed retro-modulations device can be of rugged, lightweight, compact, low-power consuming, and low cost construction, yet it can operate at elevated temperatures.        3. MEMS devices, if used, can function at high-g values owing to their small sizes and mass and their favorable mass to size ratio.        4. The retro nature of the device provides for automatic tilt error correction as well as automatic beam wander compensation (e.g., wobbling, relative platform motion).        5. The disclosed retro-modulation devices may be made very compactly using MEMS devices, for example, in combination with a variety of photonics components, such as detectors, optical elements, modulators, fibers and other integrated optical components, etc., with which the MEMS devices may be integrated and interfaced.        6. The disclosed retro-modulation devices can be fabricated using a variety of well known materials, such as Si, poly-Si, GaAs, InP, etc., and can be manufactured in high volume using commercially available technology.        
Prior art includes conventional retro-reflectors with upstream external modulators (e.g., liquid crystals, MQWs, E-O crystals). In addition, the prior art includes phase-conjugate mirrors with modulation capability (via applied electric fields or modulated pump beams). The prior art also describes optical MEMS with a modulation capability.
It has been known in the prior art that a MEMS device can be associated with a retro-reflecting corner cube. By tilting one of the mirrors of the corner cube with desired modulation information, the retro-return property of the device can be controlled in a time-wise fashion. That is, the retro-aspect of the device can be defeated temporally (i.e., the reflected light no longer returns along the reverse direction from which the corner cube was illuminated). This results in apparent modulation of the light received back at the source location, since the light either returns back to its source location or is diverted over another path, according to the tilt of one of the mirrors in the corner cube device. A drawback is that a relatively large angular displacement of the corner cube mirror is necessary to divert the backward-propagating light beyond the diffraction spread of the small MEMS device. Assuming a 50 μm scale size, a 2° tilt is required to divert the reflected light away from its diffraction-limited return path to the sender. This implies a 2 μm displacement of the mirror edge, which is relatively large, and places demands on the device geometry, the drive voltage, its slew rate, etc., of the MEMS device controlling the mirror.
The present invention improves upon this existing art by imposing the modulation via a diffractive effect (or a displacement of a very small cantilever) or via an optical Fabry-Perot effect. Each effect can be utilized in either a cat's eye or a corner cube arrangement. In the disclosed embodiments, the required mechanical displacement can be as small as 100 nm, or only 5% of what the prior art requires. This implies that the required drive voltage or electrical drive current for the present invention would be only 5% of that required for the prior art devices, thereby reducing the drive power by the square of this ratio, potentially increasing the bandwidth of the modulation from the KHz to MHz. The required mechanical displacement is a small fraction of the wavelength of the light modulated by the disclosed devices.
Also, the present invention can lead to a larger depth-of-modulation for a given drive voltage. This follows since the beam in the “off-state” is deflected over a much greater angle for a given MEMS displacement (using, e.g., the diffraction-based embodiments of this invention). The same argument also applies to the Fabry-Perot embodiments. In both cases, this reduced MEMS displacement also enables the required drive voltage to be even less for the same depth-of-modulation performance, relative to the mirror-tilt-based corner cube (the prior art). In yet a third embodiment of this invention, a small MEMS cantilever is employed at the focal plane of a lens to encode the modulation information. In this case, the MEMS device can be much smaller than the tilt device in the corner cube geometry (e.g., 10 μm versus about 50 to 100 μm), resulting in a lower voltage and torque required for the modulation encoding. The prior art also includes O-MEMS for displays (TI's DMD SLMs using MEMS cantilevers; Silicon Light Machine's diffractive-based structures), and tunable MEMS optical filters for WDM. These devices pertain to large-screen, high-definition TV multi-pixel display systems or to add/drop devices, but not to retro-communication or remote sensing devices. The prior art does not discuss or imply the possibility of using these structures as elements for retro-devices or as retro-modulation devices, nor does the prior art imply this application or even imply how they can be designed or used for the devices disclosed herein.
Examples of prior art devices are shown in FIGS. 1, 2a and 2b. FIG. 1 shows a passive retro-reflector device 24 with an external modulator 22. The construction of the retro-device can be in the form of an embossed mold with a reflective coating (in the mid-IR range), a corner cube, a three-mirror structure, lens/mirror combination, or an array of the same. A corner cube is depicted in FIG. 1 for ease of illustration. The external modulator 22 can be in the form of an electro-optic amplitude or phase modulator (e.g., a liquid crystal) or a multi-quantum well electro-absorptive modulator.
A laser 10 forms a laser beam 12 which is directed to a communications retro-reflector 20. Reflector 20 houses the aforementioned passive retro-reflector device 24 and its associated external modulator 22. Beam 12 usually will have to transit a propagation path 14, as will a beam 12r reflected by retro-reflector 20. The reflected beam 12r is modulated with data by means of modulator 22. The modulated data is detected by a detector 18 and a beam splitter 18 may be conveniently used to separate the reflected beam 12r from the incident beam 12.
FIGS. 2a and 2b show additional prior art devices. In FIG. 2a, the retro-reflector device 24 is a monolithically fabricated optical MEMS structure using reflective elements to form the required device. In this case, the mirrors of a corner cube can be deflected (by tilting them on an axis 27) to defeat the retro-directive property of the corner cube 24 (effectively resulting in a modulated return beam). In FIG. 2a, a MEMS device 26 is used to move at least one of the mirrors (or other reflector element) 24a of a corner cube reflector 24. The MEMS device is responsive to a signal on line 20s for controlling the actuation of the MEMS device 26. By putting data on line 20s, the reflected signal is, in effect, modulated since a detector 18 would only “see” the reflected beam 12r when mirror 24a is in its “normal” position. In FIG. 2a dashed lines are used to illustrate movement of mirror 24a from is “normal” (solid line) portion to its actuated (dashed line) position. When mirror 24a is in its actuated position, the beam is deflected in a direction 12d which does not permit detection by detector 18. As discussed above, the relatively large size of this MEMS-activated mirror 24a, coupled with the constraint of having to deflect the beam over an angle d in excess of the diffraction-limited spread of the retro-directed beam, limits the modulation bandwidth for a given drive voltage and slew rate. This constraint is greatly relaxed using the embodiments of the present invention.
Another example of the prior art is shown in FIG. 2b, where a phase-conjugate mirror 25 is shown with an applied modulation capability (the modulation can be externally applied as in the case of FIG. 1). The conjugator wavefront reverses the incident beam 12 and produces a reflected beam 12r, while, at the same time, the modulator encodes temporal information onto this beam.